158 DARWINISM AND HUMAN LIFE 



an individual directly induced by changes in function 

 or in environment, which transcend the limit of 

 organic elasticity and thus persist after the inducing 

 conditions have ceased to operate. Thus fattening 

 and sunbuming are modifications, though the 

 predisposition to them may be inborn ; the forma- 

 tion of a callosity as the result of pressure and 

 the reduction of a muscle by prolonged disuse are 

 modifications, though it does not, of course, follow 

 that callosities and reduced muscles may not come 

 about in a quite different way, namely, by a germinal 

 variation. Now, when we subtract from the total 

 of observed differences between members of the 

 same species all that can be described as modifica- 

 tions, we find a large remainder which we must 

 define off as inborn or germinal variations. We 

 cannot causally relate them to any pecuUarities 

 in the organism's habits or surroundings, they are 

 often distinct at birth or hinted at before birth, 

 they are rarely alike even among forms whose 

 conditions of life seem absolutely uniform. They 

 are endogenous, not exogenous in origin ; they are 

 results of changes in the germinal material ; they 

 are born, not made ; and they are more or less trans- 

 missible, though they are not by any means always 

 transmitted.' They form — at least some of them 

 form — the raw material of organic evolution, 

 whereas modifications, as defined, are probably not 

 of direct importance in evolution, since we have no 

 secure evidence that they are ever transmitted as 

 such, or in any representative degree. 



^ Darwin assumed that little fluctuations are more certainly 

 transmissible than marked idiosyncrasies, but all that we are quite 

 certain of is that a number of variations, both large and small, are 

 definitely transmissible. 



